Recent advances in broadband wireless access systems provide users with various services such as broadcasting, multimedia videos, and multimedia messages. In particular, a next-generation wireless mobile communication system is under development to provide users traveling fast with a data service over 100 Mbps and to provide users traveling slowly with a data service over 1 Gbps.
To enhance frequency transmission efficiency for providing high-rate data service, some broadband wireless access systems provide various methods for reducing control overhead. Institute of Electrical and Electronics Engineers (IEEE) 802.16m system adopting Orthogonal Frequency Division Multiple Access (OFDMA)/Orthogonal Frequency Division Multiplexing (OFDM) offers a broadcast channel called SuperFrame Header (SFH) at a fixed location of a frame so as to provide main system information and parameters.
FIG. 1 depicts the frame structure of the IEEE 802.16m system. A superframe 110 in FIG. 1 includes four frames 120. Each individual superframe 110 includes the SFH 130 in the first region. To reduce the control overhead, the SFH 130 includes Primary (P)-SFH and Secondary (S)-SFH at certain intervals. The P-SFH includes a superframe number and information for decoding the S-SFH. The S-SFH is divided into three subpackets (subpackets SP1, SP2 and SP3), and carries the main system information in different periodicities. The subpackets SP1, SP2, and SP3 are provided over one broadcast channel at different transmission intervals. The transmission periodicity of each subpacket S-SFH SPi is expressed as Equation 1.P1<P2<P3  [Eqn. 1]
In Equation 1, Pi denotes the transmission periodicity of the subpacket S-SFH SPi.
To receive each subpacket, a mobile station obtains the periodicity of the subpacket and a transmission offset. In general, since the data size encoded with one codeword is restricted according to limitation of an interleaver size of the channel encoding used in the broadcast channel, the subpackets are scheduled to be transmitted one at a time in different superframes. The periodicity of the subpacket is known, as the system information, through the SP3 of the S-SFH. Hence, the mobile station can determine the transmission point of the subpacket by calculating only the offset value of the subpacket. For doing so, the mobile station receives each subpacket for at least one time and obtains the transmission offset by applying a modulo operation to the superframe index wherein the S-SFH subpacket is received by the periodicity value of the subpacket obtained through S-SFH SP3.
The method for obtaining the transmission offset information as discussed above can be applied without error when the subpacket is transmitted in conformity with its own periodicity. However, when the system information contained in the subpackets changes according to a communication environment, the IEEE 802.16m system permits to additionally transmit the subpacket including the changed system information out of the preset intervals, in order to promptly provide the changed information. As a result, the change of the system information may require additional transmission and cause confusion in the offset calculation. In this respect, a method for preventing the confusion in the offset calculation is demanded.